The present invention relates to a noise reduction system within a passenger compartment of an automotive vehicle, by positively generating sound for canceling the noise within the passenger compartment.
There has been proposed a technique for reducing noise generated mainly by engine vibration and transmitted to the passenger compartment, by generating canceling sound from an additional sound source. The amplitude of the canceling sound is the same as that of the engine noise, but the canceling sound has a reversed phase with respect to the engine noise.
A noise reduction system of the prior art is disclosed in Japanese Laid-Open Patent Application No. 3-5255. In this prior art noise reduction system for generating canceling sound, the numerical data representative of the fundamental sine waves out of phase but in synchronism with the secondary order components of the number engine revolution are previously stored; and the phases and amplitudes of the fundamental sine waves are corrected on the basis of the number of engine revolutions detected by a crank angle sensor and the engine load detected by a pressure sensor, without directly detecting engine vibrations by any engine vibration sensor.
In the prior art system as described above, a great number of data must be stored in order to reduce various noise waveforms generated under various engine operating conditions, so that it is difficult to reduce engine vibration noise stably under various engine operating conditions. Further, since the noise generated by an engine is different according to the transmission characteristics of the respective vehicle bodies, the above-mentioned data must be stored individually according to the respective vehicles.
On the other hand, recently, another noise reduction system has been practically used, in which an LMS (least means square) algorithm is adopted on the basis of a theory such that a mean square error can be approximated by an instantaneous square error on the basis of the fact that the filter correcting equations are recursive equations, in order to simplify the calculating equations for obtaining optimum filter coefficients. Further, another noise reduction system has been put into the market, in which there is adopted an MEFX (Multiple Error Filtered X) algorithm obtained by expanding the LMS algorithm to a multichannel system. In this prior art passage compartment noise reduction system based upon the LMS algorithm, in order to reduce passage comportment noise mainly generated by engine vibration, a noise vibration source signal high in correlation to the engine vibration, that is, the primary source signal, is detected with the use of a vibration sensor; a cancel sound signal for reducing the noise is synthesized on the basis of passing the primary source signal through an adaptive filter; and the synthesized signal is generated from a speaker. Further, the noise reduction status at a noise receiving point is detected by an error microphone to obtain an error signal, and further the filter coefficients of the adaptive filter are updated in accordance with an LMS algorithm on the basis of the error signal and the primary source signal, so that the noise can be minimized at the noise receiving point.
In the above-mentioned noise reduction system using the LMS algorithm, it is possible to stably reduce noise under various operating conditions without storing a great number of data, and additionally various engine noises different from each other can be effectively reduced according to individual vehicle bodies.
In this prior art system, however, an engine vibration sensor is additionally required to detect a signal high in correlation to the engine vibration. Further, in order to obtain a primary source signal, the vibration sensor must be high in precision and reliability, thus raising a problem in that the noise reduction system is high in cost. Further, it is rather difficult to newly mount the noise reduction system on the automotive vehicle provided with no such system.
On the other hand, Japanese Laid-Open (Kokai) Patent Application No. 63-315346 discloses such a technology that engine revolution speed is detected on the basis of the intervals of the ignition signal; canceling sound previously determined for each engine revolution speed is retrieved; and the retrieved canceling sound is outputted through a speaker. On the other hand, bass sound within the passenger compartment is detected by a microphone disposed at a noise receiving position; the current bass sound is compared with the preceding bass sound; when the current bass sound is low (or high) in input level, the current canceling sound is advanced (or delayed) in phase or amplified at a high (or low) amplification factor before being outputted through the speaker, so that the bass sound detected by the microphone can be minimized.
In this prior art technology, however, since the engine revolution speed fluctuates always during vehicle traveling, and violently in particular during transient engine operation, even if an appropriate canceling sound is outputted for each engine speed range, the waveform of output of the canceling sound signal is not continuous, so that abnormal noise is inevitably produced when the canceling sound is not connected smoothly at good timing.
To overcome this problem, Japanese Laid-Open Patent Application No. 3-90448 proposes a technique for preventing abnormal sound from being generated by providing a wait time at which the canceling sound is not outputted, so that the canceling sound can be connected smoothly before and after the fluctuations of the engine speed.
In this prior art bass sound reducing technique, however, since bass sound during transient engine operation is not securely reduced, when the vehicle is started, bass sound caused by the engine is transmitted directly into the car room. In addition, when the vehicle is shifted to a constant speed travel, since the bass sound is canceled by the canceling sound generated by the speaker, there exists a problem in that the bass sound is reduced or generated according to the vehicle operating conditions and therefore the passenger does not feel pleasant.
In addition, in order to effectively reduce noise by the passenger compartment noise reduction system using the LMS algorithm, it is necessary to accurately determine the speaker-microphone transmission characteristics Cmn subjected to the influence of passenger's seat taking conditions, room temperature, room humidity, and the change thereof with the passage of time. Therefore, in the conventional method, the passenger is requested to previously determine the transmission characteristics Cmn by identifying the system after the passenger takes a seat-and before the noise reduction system is activated.
However, this operation is troublesome. Further, when random noise is generated whenever the system identification is executed, the random noise provides an unpleasant feeling to the passenger.
To overcome the above-mentioned problem, it may be possible to consider that the fixed speaker-microphone transmission characteristics can be determined in accordance with experimental results, in order to eliminate the troublesome work and the unpleasant feeling to the passengers. In this case, however, there exists another problem in that the speaker-microphone transmission characteristics deviate from the actual transmission characteristics due to the change in various environment conditions with the passage of time and the arrangement of appliances such as the cushions, accessories, child seats, etc. That is, even if the speaker-microphone transmission characteristics are once determined under some passenger compartment conditions, since the transmission characteristics vary greatly according to the other conditions deviating from the actually set speaker-microphone transmission characteristics, there exists a problem in that it is impossible to sufficiently bring the ability of the noise reduction system using the LMS algorithm to its full potential.